Classifications

• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05H—PLASMA TECHNIQUE; PRODUCTION OF ACCELERATED ELECTRICALLY-CHARGED PARTICLES OR OF NEUTRONS; PRODUCTION OR ACCELERATION OF NEUTRAL MOLECULAR OR ATOMIC BEAMS
  • H05H1/00—Generating plasma; Handling plasma
  • H05H1/24—Generating plasma
  • H05H1/48—Generating plasma using an arc
  • H05H1/484—Arrangements to provide plasma curtains or plasma showers
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05H—PLASMA TECHNIQUE; PRODUCTION OF ACCELERATED ELECTRICALLY-CHARGED PARTICLES OR OF NEUTRONS; PRODUCTION OR ACCELERATION OF NEUTRAL MOLECULAR OR ATOMIC BEAMS
  • H05H2245/00—Applications of plasma devices
  • H05H2245/40—Surface treatments

Definitions

• the present invention relates to a method for generating a curtain of gas activated with the aid of an electric discharge for the treatment of surfaces of conductive, semi-conductor, or dielectric materials, as well as a device for carrying it out. of the process.
• the surface treatments that can be performed include, among other things, sterilization, pickling, film deposition or activation of particles on the surface of the material.
• the treatment of large areas using plasma flow of elliptical or annular round section by conventional methods is carried out by scanning the surface in two directions.
• the treatment of large areas using conventional rake or curtain type flux allows scanning the surface in several directions, provided that the length of the rake or curtain is greater than the width of the surface treat.
• One disadvantage of two-dimensional scanning devices is that there is an effect of redeposition of residues on the peripheral part of the surface to be treated due to a certain migration of residual products, such as microorganisms, oils and fats, and photoresist products in areas surrounding the treatment.
• uncontrolled and repeated heating of the peripheral zones and the passage of the surface to be treated under an unconditioned plasma may cause either annealing of the material, or a partial treatment or incomplete physico-chemical transformations of the surface to be treated .
• These disadvantages are also present in the processes where a scanning by rotation is carried out which, due to the different linear scanning speeds at different diameters, have the additional disadvantage of the different durations of local treatment.
• a uniform treatment can only be obtained statistically, by carrying out numerous scans on the same area, which excludes a precision surface treatment of the material.
• Another problem with conventional devices is that a certain amount of metal vapors from the electrodes is present in the plasma jet and contaminates the treated surface. Most applications do not allow the presence of metallic additions in the plasma. from 0.0001 to 0.001%. This is the case, for example, of applications in the fields of electronics, space devices, catalytic devices.
• Conventional means for reducing the metal vapors include optimizing the material of the electrodes, and of the generating gas, in decreasing the current and increasing the voltage of the discharge, in condensing the metal vapors on the walls.
• a method and device for generating a uniform plasma cord as described in patent application WO 99/46964 or as proposed and used for the sterilization of surfaces (see P. Koulik, S. Krap ⁇ vina, A. Sa ⁇ tchenko , M. Samsonov, Vide N ° 299. Volume 1/4, 2001, ⁇ .117), consists in generating a plasma in thermodynamic equilibrium in a dielectric channel formed on the one hand, by a set of diaphragms isolated from each other and in which a passage is practiced in the form of a gutter, and on the other hand by the surface to be treated. It's important to emphasize that the surface to be treated, itself takes part in the stabilization of the arc. If the stabilizing channel thus formed is cylindrical, the treatment along the axis of the stabilized channel is theoretically strictly uniform. The treatment of a large area is done by scanning. In this case, the treatment over the entire surface is theoretically uniform.
• the above method has two disadvantages, however.
• the first is that only dielectric surfaces can be treated. Since the plasma is under long tension, experience shows that part of the current passes through the body to be treated if it is weakly conductive (for example a silicon wafer). This results in an alteration of the surface to be treated, for example due to the formation of microcraters due to micro-plates in the areas where the potential difference between the plasma and the body to be treated is greatest.
• the second disadvantage is that the surface of the body to be treated is part of the stabilizing channel and that thereby the slightest irregularity of the surface to be treated or the slightest variation in the section of the stabilizing channel, in particular during the sweeping movement, or even the less instability over time, cause a variation of the current and therefore of all the parameters of the plasma. This system is therefore unstable and obtaining uniform surface treatment under these conditions is practically impossible.
• an object of the invention is to provide a surface treatment method allowing efficient and uniform treatment of large surfaces to be treated, as well as a device for implementing the method.
• a surface treatment method and a device for implementing the method which is versatile, in particular which makes it possible to carry out various surface treatments such as sterilization, pickling, film deposition or surface activation, and which makes it possible to treat different materials such as dielectric, semiconductor, conductive, liquid or solid materials. It is also advantageous to carry out a surface treatment process which can be used for powder formation. It is advantageous to carry out a surface treatment method and a device for implementing the method which makes it possible to avoid the deposition of undesirable chemical components, such as metal vapors coming from electrodes.
• a method of surface treatment or of forming powders on a surface comprises the formation of an electric arc of stabilized plasma, the generation of a jet of activated gas in the form of a curtain from said electric arc essentially perpendicular to the axis A of said electric arc and the projection of the activated gas curtain on the surface to be treated, characterized in that the distance L separating the axis A of the electric arc from the surface to be treated is adjusted that the speed v of the activated gas flow so that the particles of the activated gas curtain are electrically neutral and in a metastable state at the time of contact with the surface to be treated.
• the distance L separating the electrical axis from the surface to be treated and the speed v of the activated gas flow are adjusted in order to satisfy the relation IJV>% where ⁇ is the relaxation time of the metastable states of the particles.
• the parameters of the activated gas flow and the scanning speed are also adjusted in order to satisfy the following relationship: the diffusion length D of the molecules, radicals and activated atoms contained in the gas curtain activated is greater than the thickness ⁇ of the boundary layer separating the incident activated gas from the surface to be treated.
• the above relationship is obtained empirically by tests, by recording different process parameters, such as the speeds, densities and temperatures of the activated gas flow and of the gas flow entrained by the surface to be treated.
• one of the means according to the invention is to regulate the speed of the gas composing the curtain of activated gas, so that it is greater than the ratio of the distance between the central axis of the electric arc and the surface to be treated at the relaxation time of the particles making up said curtain of activated gas.
• the present invention makes it possible to create large temperature and concentration gradients of the components of the activated gas on the surface to be treated.
• the method according to the invention makes it possible to ensure access to the surface to be treated by diffusion not only of excited molecules and radicals, but also of excited atoms, which greatly widens the fields of application of the process according to the invention, especially in electronics.
• the stabilized electric arc is created by a device comprising diaphragms isolated from each other which form a complex section channel (cylindrical, square, rectangular, triangular and others), having one or more inlets to access it, essentially perpendicular to Tax of this arc, a uniform flow of treatment gas along the axis of Tare.
• treatment gas is meant filler gas for the creation and maintenance of the plasma electric arc as well as for the generation of activated particles and, where appropriate, a reactive gas for carrying out a film deposition or another reaction. chemical with the surface to be treated.
• This treatment gas is activated upon contact with the stabilized arc and leaves the channel through an outlet passage which can be in the form of a slot parallel to the arc so that the resulting flux is a gas curtain activated.
• the gas or gas mixture forming the treatment gas, the speed of the activated gas flow, the distance separating the axis of the electric arc from the surface to be treated as well as the scanning speed of the surface to be treated are chosen and controlled. to make so that the activated gas is out of thermodynamic equilibrium, or in other words metastable, and uniform in a direction parallel to the electric arc which activated it.
• This activated gas curtain is therefore projected onto the surface to be treated, the relative scanning movement allowing uniform treatment over the entire surface to be treated.
• the activated gas forming the curtain does not include electrically charged particles and therefore does not conduct electricity, there is no interference between the treated surface and the stabilized arc.
• the treatment is therefore stable and independent of the state, properties (especially dielectric), movement and position of the treated surface.
• Said activated gas curtain can be created at subatmospheric (vacuum) or superatmospheric pressures. Its optimal application is however made at atmospheric pressure.
• An important feature of the present invention is that the flow of process gas, upon contact with the stabilized plasma arc, the temperature of which can be very high (e.g. 25-30.10 S K) is activated by photo effects - activation, and activation by inelastic collisions with the high energy particles of the plasma, in particular with the peripheral electrons, whose temperature is higher than the temperature of heavy particles (atoms ions).
• the construction of the activated gas curtain generator is designed so that the flow of treatment gas Qi upstream of the stabilized electric arc tangentially reaches the arc through one or more longitudinal slots, so as to bypass the core strongly ionized and high arc temperature.
• the treatment gas stabilizes the arc, helping to increase its temperature, since it contracts the section of the arc through which most of the electric current passes.
• the treatment gas is activated by convection, by photoactivation, and by transfer of energy to its particles on the part of the peripheral particles of the plasma electric arc, in particular high energy electrons. In this way, the gas flow is brought into a metastable state of thermodynamic non-equilibrium. This state has a relatively short life (relaxation).
• the speed of the gas flow must be chosen so that it is sufficiently S large so that the flow of activated gas can reach the surface to be treated without losing its activation.
• Another important characteristic of the invention is that the flow of activated gas is not ionized (ie conductor of electricity). This condition can be achieved by ensuring that the flow of process gas essentially only contacts the peripheral area of the arc poor in charged particles.
• the presence of electrically charged particles in the activated gas curtain is to be avoided for two reasons. The first is that an electrically charged particle has a high cross-section of elastic interaction with a neutral particle, which contributes to making it lose its activation energy before its contact with the surface to be treated, The second is that the particles electrically charged give electrical conduction to the activated gas curtain with all the harmful consequences mentioned above.
• an important condition for the realization of (the present invention is that the time of passage of the particles of the activated gas curtain from the electric arc to the surface to be treated, is less than the relaxation time ⁇ of the activated particles.
• the activated gas curtain is in a metastable state (out of thermodynamic equilibrium) when it comes into contact with the surface. This means that the particles bring to the surface to be treated not only their thermal energy but especially their activation energy. This makes possible chemical reactions between the particles of the surface and the activated particles of the gas curtain in the metastable state which cannot be carried out in the case of the flow of hot gas in thermodynamic equilibrium,
• a stream of complementary process gas Q 2 is brought into contact with the stream of process gas activated by the arc, and this downstream of the arc .
• the flows of complementary treatment ga ⁇ are organized so as to vary the temperature level of the activated gas curtain, but above all its activation level and its chemical composition.
• the flow of complementary treatment gas can be brought into contact with the flow of gas activated by lateral channels, most often in the form of longitudinal slots formed in the body of the curtain generation device, or formed by added nozzles.
• a very effective means of supplying the complementary treatment gas Q 2 is to introduce it into the gap existing between the surface to be treated and the body of the curtain generation device.
• the pickling, cleaning, sterilization and film deposition treatments are, in this case, extremely effective, and open up new possibilities for technology in specific fields such as the treatment of semiconductors, glass and polymeric materials.
• a major advantage of the present invention is that the pickling, cleaning and film deposition treatments can be carried out cold, ie without significant heating of the surface to be treated, just by exploiting the activation energy. incident particles brought to the surface to be treated by the activated gas curtain.
• FIG. 1 is a simplified perspective view of a surface treatment device according to the invention
• Figures 2a to 2h are sectional views of different embodiments of a device for surface treatment according to the invention.
• FIGS. 3a to 3d are also simplified section views of different embodiments of a device for the surface treatment according to the invention.
• a device for implementing a method for treating a surface to be treated 2 of an object to be treated 4 comprises a device 6 for generating an activated gas curtain 8.
• the device for generation of an activated gas curtain 6 comprises a body 10 comprising a stabilizing channel 12 for guiding and stabilizing an electric arc of plasma 14, one or more inlet conduits 16 for treatment gas Qi in communication with the stabilizing channel 12 via a gas collecting channel 18, and an opening, passage or outlet nozzle 20 for activated gas in communication with the stabilizing channel 12.
• the body 10 can be formed of stabilizing lamellae or diaphragms 22 juxtaposed, for example, in a material that is a good thermal conductor, such as a metal provided with an insulating layer to electrically insulate the slats from each other.
• a cooling system such as a water circuit (not shown) can be provided in the body 10 in order to cool the body taking into account the very high temperature of the plasma electric arc.
• the device also comprises positive 24a and negative 24b electrodes for generating the electric arc, the electrodes being connected to a source of electric current 26.
• the device 6 can also be provided with an electric field generator 42 (see Fig. 3c) to position the electric arc.
• the treatment device can furthermore comprise a kinematic system for moving the object to be treated 4 relative to the plasma generation device 6 for carrying out the scanning movement of the plasma curtain 8 on the surface to be treated 2 (the kinematic system n 'is not shown).
• the plasma generated by the electric arc 14 initiated between the electrodes 24a, 24b is stabilized and oriented parallel to the surface to be treated 2 by the wall of the channel stabilizer 12 formed by the metal strips 22 electrically isolated from each other and by a flow of treatment gas Q> ⁇ directed essentially perpendicular to the axis of the stabilizing channel and therefore of the electric arc.
• the treatment gas is distributed uniformly over the entire length of the plasma cord, in such a way that the resulting flow of activated gas is directed towards the surface to be treated 2 of the body to be treated 4, which is mounted on a support 28 propelled by a translational movement mechanism 30, ensuring the sweeping of the surface to be treated by the activated gas curtain 8.
• a device 32 generating acoustic or ultrasonic waves is optionally mounted on the support to generate vibrations of the surface to be treated 2, which makes it possible to carry out an anisotropic treatment of said surface.
• anode and cathode electrodes 24a, 24b are arranged at angles other than zero relative to the central axis A of the plasma electric arc.
• the electrodes 24a, 24b can be located in airtight pockets (not shown) ensuring equal pressures in the anode and cathode regions as well as in the region of the electric arc, so as not to disturb (in a perpendicular direction at axis A of the arc) the flow of activated gas so as not to alter the uniformity of the parameters of the activated gas curtain.
• a system of hermetic seals between the diaphragms with stabilizing lamellae 22 ensures the absence of gas currents in a direction other than the direction perpendicular to the axis A of the electric arc, which contributes to condition the longitudinal uniformity of the activated gas curtain.
• the gas or gas mixture inlet conduits in the device for generating the activated gas curtain are advantageously carried out via the collecting channels 18 intended to equalize the static pressure of the gases before and after their passage through the arc stabilized electric 14 and thus ensuring the uniform distribution of these gases over the entire length of the gas curtain activated 8.
• the entry of the treatment gas into the stabilizing channel 12 can take place either through porous walls 36 as illustrated in FIGS. 2g and 2h, or by narrow slots 38 as illustrated in FIGS. 2a to 2f .
• the supply of the stabilizing channel 12 is via an inlet in the form of a vertical slot, in these cases central, while in the embodiments of the figures 2c, 2e and 2f, the supply takes place via lateral slots 88, that is to say having an angle greater than zero with respect to the direction of the gas flow from the activated gas curtain and / or offset from the vertical plane passing through Tax A of the arc, made on either side of said vertical plane.
• the body can be provided with several lateral slits on each side of the arch, distributed around the stabilizing channel.
• the lateral slits do not necessarily have to be positioned symmetrically around the stabilizing channel, depending on the profile of the eastern channel and the position of the outlet slit.
• an electric arc of plasma is ignited between the electrodes 24a, 24b and is stabilized by the walls of the stabilizing channel 12 and a flow of treatment gas Qi.
• the surface to be treated is mounted on the mobile support 28 to effect a relative sweeping movement relative to the activated gas curtain 8.
• the treatment gas is introduced into the stabilizing channel 12 through the lateral and / or central inlet slots 38, 39 or through pores 36 on the side opposite the surface to be treated. This, partly passing through the peripheral zone of the plasma electric arc and partly bypassing the arc, heats up, activates and exits in the form of a curtain of activated gas towards the surface to treat through the opening or the outlet passage 20.
• the outlet passage 20 may be in the form of a slot of a determined width.
• the width of the slit is preferably less than the diameter of the plasma electric arc (14) in order to form a thin curtain of activated gas, the parameters of which can be well controlled.
• a thin exit slit also helps to properly confine and stabilize the plasma arc.
• the lateral slots 38 for the introduction of the treatment gases Qi into the stabilizing channel 12 are very advantageous since they make it possible to confine the plasma electric arc on the one hand, and to determine the proportion of gas passing through the zone plasma arc device and the proportion of gas bypassing the arc to adjust the composition and density of active particles of the activated gas curtain.
• the porous walls for introducing treatment gas around the arc as shown in FIGS.
• the treatment to be carried out influences the process parameters, such as the contact time, the plasma temperature, the relative speed of movement, the distance between the center of the plasma electric arc and the surface to be treated and the gas composition.
• Plasma temperature from 10,000 to 30,000 Kelvin
• Plasma speed from 10 to 1,000 m / s. (up to the speed of sound for the given plasma temperature).
• Plasma composition inert gas, oxidizing, reducing medium, chemically active gases for the synthesis of complex products, ultradisperse powders.
• Plasma purity absence of unwanted additions, in particular metal vapors.
• complementary treatment gas we mean gases Q 2 intended possibly to cool the flow of activated gas without deactivating it, or to decrease it, possibly, the electrical conductivity, or to change its chemical composition (introduction of active gases) or intended the deposition of films (introduction of ultra-dispersed powders or vapors of organic or organometallic or inorganic elements).
• the parameters of the treatment are adjusted in the device of the present invention by an appropriate construction of the stabilizing channel, and of the gas introduction and evacuation scheme.
• the body 10 of the curtain generation device is made of metal having good electrical and thermal conductivity. To avoid short circuits, it consists of diaphragms, electrically isolated from each other.
• the stabilizing channel 14 can be of half-round section ( Figures 2a and 2g), round ( Figure 2b), triangular ( Figure 2c), square ( Figures 2e and 2f) or of compound shapes ( Figure 2d). These variants correspond to different means of manufacturing the body 10 and the stabilizing channel 12,
• the gases or mixtures of complementary treatment gases Q 2 must be introduced more at the beginning of the formation of the activated gas curtain (FIGS. 2a, 2e, 2f) or also downstream of the flow (FIGS. 2b, 2c, 2d), or directly along the surface to be treated ( Figures 2c, 2d, 2h).
• the embodiment used for this example corresponds to that shown in Figure 3a.
• This embodiment allows the superficial fusion of large surfaces of refractory materials such as bricks of dimensions 350 x 150 x 30 mm.
• the body of the device as in all of the following examples, comprises cooled metal diaphragms, the thickness of a diaphragm being 6 mm.
• Argon flow, used as filler gas 5l / min.
• the mentioned process is a surface activation treatment: We use a powerful treatment but weak hydrodynamic flows to avoid é aboussures of the superficially fused material. In this case, the width of the curtain of activated gas at the location of the treatment is 5 mm. The uniformity of treatment over the entire length of the material is ⁇ 10% and is determined by the manufacturing parameters of the refractory material and its initial porosity.
• the material is treated by a free arc (not stabilized) pressed against the surface to be treated by a magnetic field, the arc being in contact with the surface to be treated.
• the surface to be treated is not in direct contact with the electric arc but with the curtain of activated gas.
• the quality of the treatment and the uniformity obtained are superior in the case of the present invention, thanks to the exclusion of the axial flows of heat of a co ⁇ vective nature, the helical instabilities of the arc, and the mass transport along the 'arc, elements which lead to the redeposition of residual products and therefore to the variations of the properties of the treated surface, along the direction of the arc.
• Example 2 Treatment of electrically conductive materials
• FIG. 3b The block diagram of the construction of the device used is shown in Figure 3b. This embodiment is used to deposit dielectric layers on a roll of aluminum foil 120 mm wide and 0.1 mm thick.
• the parameters of the device are as follows:
• Arc current 150 A Voltage 3.5 V
• Sweep speed (roller winding speed): 1.9 m / sec
• Thickness of the silicon oxide layer formed 50Q ⁇
• a film deposition process (SiO 2 ) is carried out from a stabilized arc of plasma at high temperature at atmospheric pressure, continuously over a large conductive surface.
• Argon as a component of the process gas, is used to transport small doses of reactive gases, such as oxygen and hexamethyldisilasane vapors, to slow (or even eliminate) the volume formation of SiO 2 powders, and to cool the plasma, without losing the excitation energy of molecules and radicals, to temperatures in the region of 3-4 10 a K for which the plasma has an electrical conductivity so low that there is no danger of short circuit between the electric arc and the treated metal.
• reactive gases such as oxygen and hexamethyldisilasane vapors
• the result of this application is the obtaining of a uniform passivating layer of silicon oxide SiO ⁇ of the thickness of 0.05 ⁇ m on the surface of the aluminum sheets.
• This layer has excellent adhesion, is resistant to humidity and corrosion and has a good dielectric quality. It advantageously replaces the lacquer layers conventionally used which are not very solid, sensitive to humidity and of low dielectric quality.
• FIG. 3c shows the diagram of the device for the treatment of fabrics made of organic fibers (for example polyester).
• the purpose of the treatment is to modify the structure of the fibers and activate the hydrophilic (or hydrophobic) functions over the entire surface of the fabrics at speeds acceptable by the textile industry for the large-scale processing of its products.
• the exit slit 38 ′ is produced in the form of a labyrinth so as to exclude the irradiation of the surface to be treated by the ultra-violet rays coming from the discharge, since it is known that the ultra-violet rays violets reduce the solidity qualities of synthetic materials and transform their color.
• the body of the device is made up of two halves.
• a 0.2 Tesla magnetic field was added to maintain the arc at the desired distance L from the surface of the tissue to be treated.
• the result of the treatment is an activation of the surface and a significant increase in its hydrophilic properties.
• a fabric In combination with certain chemical elements introduced into the zone of contact of the activated gas curtain with the surface to be treated in the form of a flow of complementary treatment gases Q 2 , such as for example C 3 F 6 , a fabric was obtained substantially. hydrophobic (wettability angle around 170 °) which is resistant to washing.
• the gas C 3 F 6 is introduced upstream of the contact of the activated curtain with the fabric through the longitudinal slot between the body 10 of the curtain generation device and the fabric.
• Figures 1 and 3 show the diagram of the device for pickling the silicon wafers of the photoresist (photoresist stripping) used during photolitography operations, in Electronics.
• the silicon wafers tested had a diameter of 200 mm.
• the thickness of the photoresist layer was 0.3 ⁇ m.
• the parameters of the device are:
• Width of the outlet gap of the activated gas curtain 2 mm
• the photoresist can be stripped regardless of the degree of annealing.
• Figures 1 and 3d show the diagram of the device for the etching on silicon wafers of silicon oxide through photoresist masks. This operation is used during photolithography operations in electronics.
• the treated silicon wafers had a diameter of 200 mm.
• the wafer was fixed to a support subjected to the action of an ultrasonic generator so that the vibrations of the surface to be treated take place in the direction perpendicular to the surface of the silicon wafer.
• the distance between the photoresist masks was 0.1 ⁇ .
• Stabilizer channel diameter 5mm
• Trenches with practically vertical walls were obtained, 1 ⁇ m deep and 0.1 ⁇ m wide. This result is of great interest for electronic applications, for example pickling for conventional vacuum plasma systems gives a degree of a ⁇ isotropy below 30, therefore much lower than the degree of anisotropy obtained by the process according to the invention.
• the claimed method and devices can be used for the manufacture of powders and, in particular of submicron and nanometric powders.
• the uniform distribution of the parameters of the activated gas curtain makes it possible to obtain an identical formation of clusters and powders at the different locations of the curtain and therefore to obtain good selectivity in producing the powder with minimal dispersion. its grain size, grain dimensions and their properties.
• Polycrystalline SiO 2 powders with a grain size of 10Nm ⁇ 10% were formed on a support in the form of a caterpillar conveyor. Uniformly over a width of the conveyor of 20cm.
• the incident activated gas which serves as a reactive agent is out of thermodynamic equilibrium since the condition v> U ⁇ is fulfilled.
• the L ⁇ values of all the examples are less than or equal to 10 "4 sec, which is a characteristic relaxation time of the states of the particles of the activated gas curtain, which demonstrates the metastable nature of the active particles in these examples It must remain so inside the boundary layer which separates the incident gas from the surface to be treated.
• the diffusion length D of the molecules, radicals and activated atoms contained in the flow of gas activated during their passage through the boundary layer between the incident flux and the treated surface must be greater than the thickness ⁇ of the boundary layer, as shown in the following estimate.
• the Lewis, Prandtl and Sch idt numbers are of the order of unity and the thicknesses of the boundary layer with respect to the phenomena of thermal conduction, diffusion , and viscosity are substantially equal.
• the thermal conduction coefficient ⁇ of the gases activated at atmospheric pressure, and for an incident flux temperature of ⁇ 10 4 K, is of the order of 1 W / m. Of degree.
• the thickness of the boundary layer is therefore: ⁇ ⁇ ⁇ / q ⁇ 10 m
• the diffusion length D is estimated as: D "1 / nQ ⁇ 1Q" a m where n is the density of the active particles of the incident flux, ie 0 23 m ⁇ and Q is the cross section of the inelastic interactions (ie deactivating interactions). This is less than O ' ⁇ m 2 for most molecules and radicals and even excited atoms (see LS Polak, Physics and chemistry of low temperature plasma. Naouka, Moscow 1971, ⁇ . 344).

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